Check if bone is manipulated by constraints
All constraints on the active pose bone, 

import bpy
context = bpy.context
pbone = context.active_pose_bone

for con in pbone.constraints:
    print("Constraint:", con.type)
    target = getattr(con, "target", None)
    if target:
        print("Target:", target.name, con.subtarget)
    print()


Other ways to access pose bones of an armature object

# armature object has the pose.bones collection
arm_obj = bpy.data.objects.get("Armature")
if arm_obj:
    #bone named "Bone"
    pbone = arm_obj.pose.bones.get("Bone")
    #items() gives key value pairs
    for name, pbone in arm_obj.pose.bones.items():
        print(name, pbone)